1. Field of the Invention
This invention relates to a control system for a cylinder cut-off internal combustion engine, and more particularly to a control system for a cylinder cut-off internal combustion engine in which fuel supply to some of the cylinders is cut off during running control such as cruise control.
2. Description of the Related Art
In a conventional multi-cylinder internal combustion engine having a plurality of cylinders, it has been proposed reducing fuel consumption by switching engine operation based on the engine load, between a full-cylinder operation in which all of the cylinders are supplied with fuel to be operative and a cut-off cylinder operation in which some of the cylinders are cut off with fuel supply to be inoperative. In this type of cylinder cut-off internal combustion engine, since shock may sometimes be generated by torque fluctuation during engine operation changeover, it has also been proposed eliminating shock by adjusting the throttle opening during a transitional period of changeover, as taught in Japanese Laid-Open Patent Application H10 (1998)-103097, for example.
It is also known to control a vehicle to cruise at a desired vehicle velocity set by the operator. A similar preceding vehicle follow-up control (so-called “adaptive cruise control”) is known in which a vehicle is controlled to detect the distance between itself and a preceding vehicle using radar or the like, and to travel while maintaining a desired inter-vehicle distance between itself and the preceding vehicle. In these types of control, the vehicle velocity or inter-vehicle distance when the operator manipulates a corresponding setting switch is stored as a desired velocity or distance, and the throttle opening is adjusted via an actuator such that the vehicle runs at the stored desired velocity or the vehicle runs at a desired velocity required to maintain the stored desired inter-vehicle distance between itself and the preceding vehicle, as disclosed, e.g., in Japanese Laid-Open Patent Application H9 (1997)-290665.
Typically, in the running control described above, when the vehicle velocity drops, the throttle opening is adjusted more finely and more promptly than a case that an operator operates an accelerator pedal manually. Accordingly, when the vehicle velocity falls during the changeover from the full-cylinder operation to the cut-off cylinder operation, or when the vehicle velocity falls due to a change in the gradient of the road during the cut-off cylinder operation is in progress, the throttle opening is adjusted quickly.
In the former case, however, since the engine torque is insufficient, the intended change in vehicle velocity cannot be obtained and hence, the throttle is likely to be opened excessively. In an internal combustion engine during the cut-off cylinder operation, meanwhile, the cut-off cylinder operation is canceled if the torque is insufficient, when it is determined that the operator intends to accelerate from changes in the throttle opening and the accompanying manifold pressure.
It is therefore preferable to control the throttle opening in a closing direction during running control so as to prevent frequent switching between the cut-off cylinder operation and the full-cylinder operation in such a way that the cut-off cylinder operation is continued as long as possible, whilst switching to the full-cylinder operation is only performed after the vehicle velocity has fallen. In doing so, however, when the vehicle velocity returns to normal, sharp or drastic acceleration (increase in velocity) accompanying the torque fluctuation, may sometimes occur that.